1. Field of the Invention
The present invention relates to a field emission microcathode array, and particularly to an improvement of a field emission microcathode array involving emitter cones for emitting electrons. The emitter cones are grouped into small blocks so that, even if an electrode-to-electrode short circuit occurs in any one of the small blocks, the failure will be confined to the small block in question, thereby improving the reliability of the array as a whole. The present invention also relates to a field emission microcathode array involving a gate electrode having openings of different sizes to expand the operation margin.
2. Description of the Related Art
Field emission microcathodes are essential for vacuum microdevices such as very small microwave vacuum tubes and display elements.
FIGS. 1(A) and 1(B) illustrate a structure of a field emission microcathode, where FIG. 1(A) is a perspective view and FIG. 1(B) is a sectional view.
In the figures, a substrate 1' is made of, for example, a semiconductor. A cone 2' serving as an emitter is formed on the substrate 1'. A tip 20' of the cone 2' is surrounded by a gate electrode 30. The substrate 1' is separated from the gate electrode 30 by a gate insulation film (not shown). A gate opening 3 is formed around the tip 20' of the cone 2'. Operational characteristics of this field emission microcathode are mainly determined by the radius Rg of the gate opening 3, the height Ht of the cone 2', and the thickness Hg of the gate insulation film.
The semiconductor substrate 1' serves as a cathode electrode. This substrate may be made of insulation material and a cathode electrode made of a conductive film may be disposed between the substrate and the cone. Usually, these elements are made several micrometers or smaller in size by photolithography which is known in the field of semiconductor ICs.
When a negative voltage is applied to the cone 2' and the gate electrode 30 is positive, the tip 20' of the cone 2' emits electrons. Namely, the cone 2' acts as a field emission microcathode.
Although the example of FIGS. 1(A) and 1(B) involves only one emitter cone, a plurality of cones may be arranged in an array on a single substrate, and FIGS. 2(A) and 2(B) are examples of such a field emission microcathode array forming a display; more specifically, FIG. 2(A) is a sectional view showing an essential part of the display and FIG. 2(B) is a schematic view explaining a method of driving the display.
In FIGS. 2(A) and 2(B), the field emission microcathode array 50' comprises many field emission microcathodes formed on a substrate 1'. The microcathodes may be arranged two-dimensionally, or in longitudinal and lateral rows, to form an X-Y matrix on the substrate 1'.
The field emission microcathode array itself is already known. It may be made in sizes and pitches disclosed by the present inventors (Institute of Electronics, Information and Communication Engineers of Japan, Autumn National Convention, 1990, SC-8-2, 5-28-2).
Opposite the field emission microcathode array 50', there is arranged a transparent substrate 10 made of, for example, glass. Anodes 12 are formed on the lower face of the substrate 10. Each of the anodes 12 is made of an ITO (In.sub.2 O.sub.3 -SnO.sub.2) film having a thickness of 200 to 300 nm and an area of 100.times.100 .mu.m. A pitch between the adjacent anodes 12 is about 30 .mu.m. On each of the anodes 12, a fluorescent dot 11 smaller than the anode 12 is disposed. The dot 11 is made of, for example, a ZnO:Zn film having a thickness of 2 .mu.m. Each dot 11 forms a pixel.
The substrates 1' and 10 are spaced apart from each other by a distance of about 200 .mu.m, to form a display panel 100.
The display panel 100 is driven by a control circuit (an anode selection circuit) 200 shown in FIG. 2(B). The anode selection circuit 200 is connected to the anodes 12. A gate power source 260 applies a gate voltage so that the cones 2' simultaneously emit electrons, which are specifically attracted by a specific one of the anodes 12 that are selected by the anode selection circuit 200. The electrons attracted by the specific anode cause the fluorescent dot 11 on the anode 12 in question to emit light.
In this way, the anode selection circuit 200 properly selects an anode 12, to which a positive potential is applied thereby to cause the fluorescent dot 11 on the selected anode 12 to emit light, thus driving the display.
FIGS. 3(A) to 3(C) show a conventional arrangement of a field emission microcathode array, where FIG. 3(A) is a perspective view, FIG. 3(B) a partially enlarged view, and FIG. 3(C) a sectional view along a line 3(C)--3(C) of FIG. 3(A).
In the figures, a substrate 1 is made of glass. A cathode 6 is formed on the substrate 1, and an insulation film 7 is formed on the cathode 6. Many cones 2 are two-dimensionally arranged on and formed in the insulation film 7. A gate electrode 30 having gate openings 3 is laminated such that each opening 3 surrounds a tip 20 of a corresponding cone 2, to thereby form a field emission microcathode array 50'.
In this example, the cones 2 are two-dimensionally arranged over the substrate 1. They may be arranged in longitudinal and lateral rows to form an X-Y matrix for each pixel (IEEE Trans. on Electron Device, Vol. 36, p. 225, 1989).
The microcathodes, each having a diameter of several micrometers, of the array 50' may be arranged at intervals of several micrometers, so that several hundreds of microcathodes can be arranged for each pixel to form an area of about 100.times.100 .mu.m. This produces a bright screen and provides good redundancy against unevenness in brightness caused by differences in the characteristics of individual microcathodes.
If the tip 20 of the cone 2 is short-circuited to the electrode 30 due to conductive dust or broken chips of the cone, a critical problem may arise, in that the emission of electrons may thereby stop, for a corresponding pixel or even over a display screen as a whole. This problem must be solved.
FIG. 4 shows models of problems that occur on the field emission microcathode array.
If the tip of a cone formed between the cathode 6 and the gate electrode 30 is broken, as illustrated in the case of a cone 2', no electron will be emitted to drive the emitter in question.
If the shape of a cone is deformed, as in the case of a cone 2", the cone will be short-circuited to the gate electrode 30 and thereby will equalize the potential of the gate and emitter. This causes all emitters to malfunction and causes an excessive current to flow through the gate electrode 30, thereby breaking the array as a whole.
To solve these problems, accordingly, an object of the present invention is to provide a field emission microcathode array that ensures continuous operation of the array as a whole even if a cone is locally short-circuited to a gate electrode.
The center of each opening 3 of the gate electrode 30 must correctly agree with the center of the tip of a corresponding cone 2 according to a conventional fabrication method. What is important is the distance between the gate electrode and the tip of the cone. If the distance satisfies certain criteria, a sufficient emission current will be obtained. If the distance is not within the criteria, the emission current will be impractically low. Namely, the diameter of each gate opening or the distance between the tip of the cone and the gate electrode must be strictly controlled.
FIG. 5 explains this issue. When the diameter of the opening 3 of the gate electrode 30 is properly set, the emission current is remarkably high. If the optimum condition is not met even slightly, the emission current becomes drastically low.
FIG. 6 shows a relationship between a gate voltage Vg and an emission current Ie with a change in the diameter of the gate opening being a parameter. In the figure, the ordinate represents the discharge (i.e., emission) current Ie, and the abscissa the gate voltage Vg. Curves (1) to (3) represent the characteristics of field emission cathodes having respective, different gate opening sizes as shown in FIG. 12, discussed in more detail hereinafter, curve (1) represents the characteristics of a field emission cathode with a middle-sized gate opening 3b, curve (2) represents the characteristics of a field emission cathode with a small-sized gate opening 3c, and curve (3) represents the characteristics of a field emission cathode with a large-sized gate opening 3a.
An optimum radius of the gate opening is Rgo. If the actual size (i.e., radius) of any gate opening is larger or smaller than the optimum, it produces a very small emission current. Namely, a sufficient emission current will not be obtained if the radius of the gate opening differs from the optimum value.
Accordingly, the area and shape of each opening of the gate electrode in the field emission microcathode array must be strictly fabricated through precise designing and process control. Even under such strict control, the diameters of openings of the gate electrode may fluctuate for some reason. In this case, the production costs of the microcathode array may increase since the production yield may decrease.
Another object of the present invention is to provide a field emission microcathode array that is free from the above problems associated with conventional techniques, sufficiently demonstrates specified characteristics, and can be efficiently fabricated with a high production yield at a low cost.